Radio frequency communication devices (e.g., cell phones) rely upon power amplifiers to convert an input signal with a small amount of energy into a similar output signal with a larger amount of energy. Efficiency and linearity are both factors in the performance of power amplifiers in modern wireless systems. An ideal power amplifier would be a totally linear device, meaning that as a drive signal to the amplifier is increased, the output signal delivered by the amplifier also increases in a corresponding manner, until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output.
In practice, there is a tradeoff between efficiency and linearity that makes it difficult to satisfy both factors at the same time. At maximum output power and highest efficiency, the amplifier linearity is often so low that only signals with a constant envelope can be transmitted. At lower output powers, the linearity increases, but the power efficiency decreases.
The linear amplification with nonlinear components (LINC) method offers a method for amplification with good linearity and high efficiency. However, the LINC method has disadvantages in that its efficiency is poor at small signal amplitudes and that it is strongly affected by impedance mismatching of the antenna.
The poor efficiency at small signal amplitudes is especially problematic for multiband or multimode communication devices. Multiband phones allow users to communicate with multiple distinct types of communication systems. Most often, the different communication systems require different modulation formats having widely varying dynamic ranges. Therefore, to adapt the transmission of data (e.g., audio, video, etc.) for multiband systems it is necessary for transmitters to switch between the various modulation methods and thus between the different required standardized transmission power levels. For example, multiband systems must switch between the high amplitude transmission required for GMS, and the low amplitude transmission that frequently occurs in the case of third generation (3G) devices and long term evolution (LTE) modulations.